The structures, distributions and functions of molecules of importance in the nervous system are being studied. Several cDNAs that encode neurotransmitter receptors have been isolated. The distributions of mRNAs to which these DNAs are complementary have been determined by means of in situ hybridization histochemistry (ISHH) as have the distributions of other mRNAs (e.g., peptide processing proteases, enzymes involved in neurotransmitter biosynthesis). ISHH has also been used to examine the effects of physiological and pharmacological manipulations on specific MRNA levels in the CNS. Expression of receptor cDNAs has allowed functional studies to be carried out--especially studies of receptor coupling to second messenger systems.